1. Field of the Invention
The present invention relates to circuits having specific, controlled signal node impedances, and in particular to circuits for maintaining such specific, controlled signal node impedances.
2. Description of the Related Art
High frequency, e.g. radio frequency (RF), circuits often have input and output signal ports with specific, controlled impedances. These impedances are typically matched to the impedance of transmission lines connecting such circuits. This allows for maximum transfer of power to or from such circuits.
Such high frequency circuits are used in many applications, including mobile wireless communications systems. In mobile, or portable, applications, such circuits are often powered by batteries. Further, such circuits often have power saving features whereby some of the circuitry is powered down when not in use. However, the remaining portion of the circuitry still in use must still see the same specific, controlled impedances at its input and output ports. Impedance changes, which can easily be introduced by those portions of the circuitry being powered down, can adversely affect the remaining circuits which are still powered up. Such adverse effects can include circuit ringing and oscillator frequency pulling.
Conventional approaches to solving this problem of maintaining a desired impedance at a signal node include: (1) placing a constant (and typically low) shunt impedance at the node of interest; (2) allowing some small amount of supply current flow during the powered down state to maintain the circuit impedance; and (3) using external impedance matching elements. However, each of these techniques introduces problems. First, keeping a constant shunt impedance at the node of interest causes loss of signal gain and increased noise due to the additional thermal noise introduced by resistive elements. Second, maintaining some amount of power supply current flow during the otherwise so-called "powered down" state tends to negate any benefits otherwise sought to be achieved by the circuit's power saving features. Third, the use of external impedance matching elements requires the use of extra components, which increases costs and complexity, particularly in an integrated circuit environment.